Fire detecting device



April 5, 1938. v. DuRBlN 2,113,296

1 FIRE DETECTING DEVICE named s, 193s UNITED STATES PATENT oFFlcE Boitier-Cabot Electric Company,

Bolton,

, Mass., acorporationoflhlachllaetts `Y Application August 3l, 1934, Serial 110,142,264

24 Claim. (Clint-188) 'I'his invention relates to heat responsive devices and `especially to fire detecting, alarm-initiating devices.

l Theiire detecting device embodying the pres- 5 ent invention is adapted to be located within the zone to be protected in thermal relation with the ambient atmosphere and so arranged as to be 'responsive to and operated by am. abnormal temperature condition of the atmosphere to control l an alarm circuit as by either making or breaking the circuit, thereby to actuate some suitable remotely located alarm device.

Since the device may remain in an unoperated condition over a long period of time and yet, to

l perform itsfunction when called upon to do so,

. must be maintained constantly in operative condition, an. object of the present invention is to enclose the operating parts o1' the device including the heat actuated element within an enclosing 20 casing that is sufllciently tight to exclude from the operating parts dust and foreign matter that might impair or prevent the operation of the device and at the same time provides an effective transfer of heat from the external atmosphere tor 25 the internal heat actuated element A further object oi the invention is the provision of a fire detecting device including a casing having therein a heat actuated element. the casing having a member of good heat conduction, as 30 a metal cover, that is exposedto the ambient atmosphere and serves to transmit heat therefrom to the heat actuated device. A further object of the invention is the provi- & sion of a ilre detecting device having a thermo- 35 static bimetal strip wound in spiral form as the heat actuating element, the element being enclosed within a casting having a cover of good heat conducting material and the element resting on edge loosely on the cover so that it can move `40 freely in response to the heating thereof and so that the heat of the ambient atmosphere is transmitted by conduction through the cover to the thermostatic strip.

Complete protection against nre in a zone 45 supervised by a device oi.' the present invention requires that the device be responsive both to a predetermined elevation of temperature and also to an excessive ratevoi.' rise of temperature regardless of the elevation thereof. Hence a fur- 60 ther object of the present invention is the provision of a re detecting device having an improvedconstruction that includes two heat actuated elements that cooperate' to respond to a predetermined elevation of temperature and also to an u abnormal rate of rise thereof.

"been a source of uncertainty.

. An additional object of the invention resides in the use of one circuit controlling device for both heat actuated elements so that the operation of either element or both conjointly-results in the proper operation oi the circuit controlling 5 device.

A yet further object of the invention is the provision ot a re detecting device having two heat actuated elements arranged to cause the device to respond both to a. predetermined elevation of temperature and an abnormal rate oi.' rise of temperature, and a casing enclosing both elements with provision whereby one element receives heat from the ambient atmosphere more rapidly than the other element and particularly wherein the casing has a part of goo'd thermal conductivity arranged in contact with one of said heat actuated elements and wherein the other element is thermally insulated to an appropriate degree Afrom the -iirst element and from the conducting part of the casing.

As has been stated above a fire detecting device of the type with which the present invention is concerned is required to stand idle for long periods of time and yet must operate satisfactorily when subjected to a dangerous temperature condition. Heretoi'ore the circuit controlling contacts have If the circuit is normally open, so that the contacts are maintained separated for a long period of time, dirt and oxide is liable to accumulate on the contact faces and preventl the proper closing of the circuit when the contacts are brought together. Ii' the circuit is normally closed, so that thecontacts are normally engaged, they are liable to stick together and will not separate when required to do so. Hence a further object oi' the invention is the provision of a. fire-detecting device wherein the circuit controller has its contacts enclosedwithinasealedcasing thatis40 evacuated or contains an inert atmosphere so that 'complete reliability of operation over indefinitely long periods is assured. Specifically the circuit controller is of the mercury contactor ,type wherein the spaced circuit electrodes are r sealed within an evacuated casing or one containing an inert atmosphere and wherein a mobile bodyofmercuryiscausedtobridgeandalso disconnect the electrodes upon the tilting of the l casing in opposite directions, the casing being tilted by movement of the heat actuated element.

A further-.object of the invention is .the provision of a nre detecting device including two heat actuated elements having movable ends on and between both of which a carrier for the contacter casing `is fulcrumed, the arrangement being such that the movement of either one element relative to the other causes the tilting of the carrier and the operation of the contactor.

A yet further object is generally to improve the construction of re detecting, alarm-initiating de- A vices in details that are important from the standpoint of reliability of operation and of economical manufacture and assembly and freedom from unauthorized manipulation.

Fig. 1 is a side elevation of the fire detecting device of the present invention in installed position.

Fig. 2 is a plan view of the device of Fig. l.

Fig. 3 is an enlarged plan view of the device of Fig. l with the cover removed.

Fig. d is a section taken along line i of Fig. 3.

Fig. 5 is a side elevation of the device with the Acasing partly broken away to illustrate the circuit controller which as here shown is in an unoperated or open circuit condition.

Fig. 6 is a sectional view similar to Fig. d but showing a modication providing increased heat insulation between the two heat Vactuated elements.

Fig. 7 is a side elevation of the device with the casing partly broken away to illustrate the man- Fig. i0 is a view similar to Fig. 9 but illustrating the position of the circuit controller when both heat actuated eiements are moved at about the same rate.

Fig. il is a View similar to Fig. l0 but illustrat- -ing the position of the circuit controller after the top heat actuated element has been arrested in advanced position and the lower heat actuated element has moved further to actuate the circuit controller.

Fig. l2 is an enlarged plan view of a portion of a heat actuated element and illustratingr the-bimetal construction thereof.

The lire detecting device embodying the present invention comprises generally a casing il containing the operative elements of the device. The device is adapted to be secured in any desired location in a room preferably on the ceiling I6 thereof and may be attached to and form the cover of a junction box i8 through which conductors 20 of an alarm circuit are extended. The casing comprises a base 22 preferably circular in configuration having a flat top face 2l and composed of some suitable electrically insulating material as a molded articial resin. The casing isV provided with a generally circular recess 26 open at the bottom and adapted to receive the operating elements of the device. The base is formed with an integral ledge 2l which projects into the recess from one side'thereof and carries sets of circuit terminals Il and 32. A plate 3l of suitable heat insulating material is secured to the top face of -said ledge 28 preferably removably by screws 36 and projects into thec recess 2l in spaced relation with the top wall 24 of the Said plate at its inner end terminates in a circular heat insulating disc 3l whichis disposed coaxially of the recess and is integral with the plate andis of suitable area and thickness to arranco provide a suitable time lag or delay in the operation of one heat-actuated element over the other, as will be explained hereinafter. A cylindrical boss 40 is integral and coaxial with and projects below said disc 38 and partially externally of the recess and constitutes a. support and anchorage for the inner end of the lower hea-t actuated element 42. Said element 42 comprises a thermostatic strip consisting of two metals 2a and 4211, see Fig. 12, having dissimilar coeillcients of heat expansion secured together back to back. Said strip is formed into spiral coniiguration and has a radially inwardly extended xed inner end 54 and a radially outwardly Iextended movable end 46. The inner end is received within a longitudinally extended slot in the boss 40 and thereby is anchored against rotation. The ilrst or innermost convolution 48 of the spiral is preferably of circular configuration so that it overlaps and presses against the inner end of the strip and thereby holds it in the slot against escape therefrom without the necessity for other securing means. The open recess 28 of the base is closed by a cover 50 which preferably is cup shaped having a flat bottom wall 52 and an upstanding clrcular side wall 54 which closely fits within the recess of the base sufficiently tightly to exclude dust and other deleterious foreign matter from entering the casing. The bottom wall 52 of the cover overlies and bears against the end face of the hub c and is secured thereto by a screw 56 which is screw-threaded in the boss, the screw having an enlarged thin head 58 which overlies the cover and has a pair of wrench receiving apertures BG therein in which the prongs of a suitable wrench can be inserted for the purpose of rotating the screw. lifter the screw has been inserted it is locked in position by a pin 62 which is passed through one of said wrench holes 60 and dri-ven into the end of the boss 40. A rating plate 66 is then sprung permanently into a cup shaped recess formed in the outer face of the head of the screw and overlies the pin 62 and permanently -prevents its removal so that the screw can not be removed subsequently by an unauthorized person. The cover 50 is locked against rotation on the screw 56 additionally to the pin 62 by a tongue 63 formed at the edge of the supporting plate 34, which tongue is located within a notch 65, see especially Fig. 7, formed in the edge of the side wall 54 of the cover, the tongue thereby acting as a key to prevent the cover from rotating and loosening the screw 5.

The cover 5U, in accordance with this invention, is composed of some metal, as copper, that'has high thermal conductivity so that the rate of heat transmission through it from the atmosphere surrounding the device is rapid. 'Ihe heat actuated element 42 has its lower face substantially ush with the top of the boss Il and hence rests upon and is in direct metallic contact with the inner face of the metal wall 52 ofthe cover 5l. The heat actuated element is suported only at its center and. due toits spiral conguration and the long length of the strip composing it, is axially flexible so that it can rest by its weight upon the cover and yet expand and contract freely while in contact with the cover. Due to the metallic contact between the element and the heatcollecting cover the heat of the ambient atmosphere is transmitted through thecover rapidly to the heat actuated element wherebyto cause it to assume an expanded condition dependent upon the temperatm'e of the atmosphere. A second heat actuated element l whichmay be and prefaliases erably is substantially identical with the lower heat actuated element 42 is disposed in the `upper part of the recess 28 of the base on the top side o! the insulating plate 84. A metal stud I8 is molded in to the disc 88 of said insulating plate and projects upwardly in the axial line of the recess 28. A metal hub 18 is received rotatably loosely on said stud and is in engagement with the inner end of the upper heat actuated element in the same way that the boss 48 engages the inner end of the lower element I2. A washer 12 is disposed on top of the stud and a nut 14 is screwthreaded thereon to exert pressure against the washer and hub andclamp the hub upon the insulating supporting plate M. 'I'he top face of the hub is provided with a pair of diametrically opposed-slots 16 and the washer is formed with corresponding slots adapted to receive projections oi' a tool to hold the hub and hence the heat actuated element 88 against rotation while the nut 1l is being rotated to clamp the hub in adjusted position. 'Ihe stud 88 and the nut 'M thereon projects upwardly through a hole 'I8 in the top wall 2l of the insulating base so that the setting of the upper heat actuated element can be made readily. The upper heat actuated element is spaced from the lower element and from the cover 58 and is insulated from both by the heat insulating discs 88 and hence receives heat by radiation from the cover more slowly than the lower element 42. When the device is subjected to a rise in temperature that is so slow that the rise is not ordinarily caused by a nre condition the temperature oi' the top element closely follows the increase in ambient temperature so that the two elements expand at about the same rate and thus do not become operative to initiate a warning signal. When, however, the ambient temperature rises so rapidly as to indicate the probability of a dangerous lire condition the lower heat actuated element becomes heated faster and thus expands more rapidly than the upper element and hence causes the control of the signal circuit to initiate a warning signal. The difference in the rate of heating of the two elements and hence the time lag of expansion between them in response to a rise of temperature can be varied by varying the thickness of the insulating supporting plate 8l and by the amount of area of said plate interposed between the elements. In Fig. 6 a modied form of plate a is illustrated wherein the disc 88a is approximately as large in diameter as the diameter of the spiral heat actuated elements I2 and 88 thereby to provide a greater time l'ag of heating andoperation of the two elements than is provided by the smaller area of the disc illustrated in Fig. 4. ,Y

'Ihe two actuated elements are arranged independently and also conjointly to control a circuit controller. Said circuit controller is of the so-called mercury contactor type and comprises an elongated cylindrical `sealed enclosing casing 88 composed oi' glass or the like having its circuit terminals 82 and 84 sealed in one end of the tube and projecting in spaced relation into the interior thereof. The tube contains a small mobile body of mercury 88 that, when thetube is inclined in one direction, occupies one end of the tube and l"is in vcontact with and electrically bridges the terminals and, when the tube is inclined in the opposite direction, leaves the terminals and runs into and occupies the other end of the tube. The

. tube is evacuated or contains a gas that is inert with respect to the mercury and the electrodes so that these elements remain clean and in operative condition indefinitely. With this type of circuit controller the proper operation thereof is assured even after years of inactivity and thus the ilre detecting device is reliable to a degree not readily attained by the usual type of circuit controller having exposed separable solid contact members. 'Ihe circuit controller is mounted in a carrier which comprises a vertically disposed insulating block 88 having a hole through the middle thereof in which the tube is frictionally retained. 'I'he top and bottom ends of the block are provided with V shaped notches 9|! in which the movable ends 48 and 82 of the lower and upper heat actuated elements respectively are located. The confronting edges oi said ends engage and fulcrum in the bottoms oi the notches and bear with a slight tension against the block by the inherent axial resiliency of the heat actuated elements due to the spiral conilguration thereof. The ends of said elements are retained in said notches against accidental displacement therefrom by retaining pins 84 which are frlctionally retained in the carrier 88 and traverse the notches and pass loosely through relatively large clearance holes in the ends 48 and 98. The circuit terminals 82 and M are electrically connected to conductors 98 and 88 which are attached to the carrier 88 and then extend loosely to the terminals 38 and 32 respectively of the device. The conductors are suiiicientiy slack between said carrier and the terminals so as to permit unrestricted angular movement of the heat actuated elements and the tilting of the controller within the required range. The angular displacement of the free end 92 of the upper heat actuated element 88 is limited for the purpose of defining the elevated temperature at which the device shall operate its circuit controller. To this end the device is provided with a stop member or arm comprising a plate |00, see Figs. 3, 4 and 8, which is capable of angular movement about the stud 88 under the hub 18 and is clamped in set position by said hub against the supporting plate M. Said plate |00 is provided with an upwardly extended end portion I 82 which bears trictionally against the inner face of the wall I6 of the base 22 by the inherent resiliency of the plate, thereby cooperating with the clanping action of the hub 18 in holding the plate in set position. The end |82 is provided with-an extension IM that is adapted to be engaged by the -free end of the upper heat actuated element or, more precisely, by the.upper end of the circuitcontroller carrier 88 whereby to limit the expension movement of the upper heat actuated element and thereby eil'ect 'the operation of the circuit controller in response to an elevation of temperature determined by the setting of said stop arm.

The operation of the device is essentially as follows: The normal position of the heat actuated elements can be ,as illustrated in Fig. 3 and in the full lines of Fig. 9. When the ambient temperature rises both heat actuated devices are influenced by the increase of temperature and are caused to expand in a direction to increase the lengths of their spirals. If the rise of ambient temperatrn'e is relatively rapid the lower heat actuated-device expands more rapidly than the upper one since it is more directly influenced by the heatwhile the upper element, being relatively insulated from the heat, expands moreslowly. IThe result ultimately is that the lower end of the carrier 88 advances more than the upper end thereof so that the upper end fulcrums upon the free end of the upper element and causes the tilting of the tube 80 in a direction to cause the mercury to run into the terminal end of the tube and bridge the terminals 82 and M and thereby complete the circuit through said terminals to eiect the initiation of an alarm signal. If the rise of temperature is so slow that both heat actuated elements expand practically simultaneously the circuit controller will be advanced in the manner indicated in Fig. 10 but without change in the relative disposition of the free ends of the elements sov that the circuit controller will not be tilted and operated. If the slow rise of temperature continues until the temperature elevation is sufficiently great, however, the advance of the free ends of the elements will continue until the upper end of the carrier 88 is brought into engagement with the projection IM of the arm |00. Further advance ofthe top of the upper element is thus prevented. The lower element, however, will continue its advance and in doing so will tilt the tube- W into the position illustrated in Fig. 11, therebyl causing the mercury body to run into the lower end of the tube and bridge the circuit terminals 82 and I4 in the manner above described, thereby initiating an alarm signal as a result of the elevation of temperature regardless of'the slow rate oi' increase of temperature.

I claim:

l. A ilre detecting device including a pair of thermo-'responsive elements. a dust tight casing enclosing said elements and isolating them from the ambient atmosphere, said elements being equally freely exposed to the atmosphere within said casing, and a signal initiating device associated with said elements, said casing having a heat-transmitting wall exposed to the atmosphere surrounding said casing and adapted to become heated by exposure to the atmosphere and heat the interior of said casing, and one of said elements being disposed nearer than the otherl element to said wall so that it is more immediately responsive to th'e atmosphere externally of said casing.

2. A fire detecting device including an enclosing casing having a heat conducting wall exposed to the atmosphere vsurrounding said casins, a pair of heat responsive elements located within said casing one adjacent and the other remote from said heat conducting wall and receiving heat from the atmosphere only by transmission through the material of said wall. said elements being equally freely exposed to the at,-

mosphere within said casing. heat insulating means carried by the casing independently of and interposed between said elements, and signal initiating means associated with said elements.

3. A fire detecting device comprising an enclosed casing having avheat conducting wall ex posed to the atmosphere externally of the device for transferring heat from the exterior to the interior of the casing, a-pair of thermo-responsive elements located within said casing and both being approximately equally freely exposed to the atmosphere within the casing, one of saidl elements being adjacent and the other remote from said wall, and a heat insulating barrier interposed between said wall and saidmore remote element.

4. A fire detectingl device comprising a support having a projecting stud,l a hub angularly movable on said stud having a longitudinally directed slot in its outer face, a thermostatic spiral surrounding and carried by said hub having a free outer end and an inwardly directed end that is located in said slot, said slot opening into the end face of said hub, a' washer on said stud upon said end face and overlying said slot, and a clamping nut screw-threaded on said stud and bearing on said washer.

5. A ilre detecting device as deiined in claim 4, said hub and washer being larger in diameter than said nut and having registering wrench engaging notched portions by which said hub and the thermostatic spiral carried thereby can be shifted angularly on said stud whereby said spiral and nut may be relatively rotated while one is held stationary.

6. A ilre detecting device comprising a support having a projecting stud. a thermostatic spiral having an inner hub rotatable on said stud, a clamping nut screw-threaded onA said stud and bearing on said hub, said spiral having a free outer end, signal initiating means associated with said outer end, and a limit arm located between said support and hub and angularly shiftable on Said stud having an end disposed in the path of travel of the free end of said spiral.

'1. A nre detecting device comprising an enclosing casing having a recess, a support in said re'- cess having a boss outstanding from said support in one direction and a stud outstanding from said support in the opposite direction, a thermostatic spiral carried by said boss, a second thermcstatic spiral having a hub rotatable on said stud. said spirals having free outer ends, a circuit controller associated with said free ends, and a reallient limit arm pivoted on said stud between' said hub and support having a free end bearing yieldingly upon and movably over the bottom of said recess in the path of movement of the free end of one of said thermostatic spirals to limit the movement lof the free end of one only of said spirals, whereby to eifect the operation of said Icircuit controller, and a nut screw-threaded on said stud which clamps said arm in set position between said support and hub.

8. A nre detecting device comprising a support. a' thermostatic spiral having its inner end pivotj ally mounted on said support and having a free outer end, a limit arm pivotaliy mounted on said support concentrlcally with said spiral having asend portion dispom in the path of movement of the free end of said spiral, clamping means for securing both said spiral and said arm in predetermined adjusted podtions. means permitting angular adjustmenttof said arm and spiral each without ent of the other, and signal in itiating 'means associated with the free end of said spiral.

9. A iire detecting device comprising a supporting plate. a support therefor engaged with one end of said plate, the other end of said platehaving a. bou projecting in one direction laterally therefrom and a projecting stud that is co-axial with said boss and projects in the opposite direc v and has a boss, a thermostatic spiral in said recess on said boss and having a free outerxend, a metal cover plate for said recess parallel toand in contact with the edge of the convoluticns of said spiral, means securing said plate in the aforesaid relation to said b'oss, and signal initiating means associated with the free end of said spiral.

11.'A thermo-responsive device comprising two thermo-responsive members responsive at diierent rates when subject to the same operating temperature and movable in the same direction by substantially the same amountwhen the operating temperature is slowly changing and movable by substantially different amounts when the operating temperature is rapidly changing, and a circuit controller associated Ywith said. members and movable with both of them and also with any one of them, saidcircuit controller having means by which its movements with both members moving in the same direction by the same amount is ineffective to control a circuit and by which itis eil'ectively operated only when the position of one member changes by a predetermined amount with respect to the other member.

12. A ilre detecting device comprising two movable thermo-responsive members responsive at diierent rates to a common temperature, a carrier disposed between and iulcrumed on both 'members and arranged to tilt when one of said members becomes displaced more than the other member and to be bodily displaced without tilting when both members are displaced by the same amount in the same direction, and a mercury contactor carried by and tiltable with said carrier.

13. A nre detecting device comprising two thermostatic strips having iixed ends and free ends that are movable in the same direction when said stripsare subjected to elevated temperalture, a tiltable carrier disposed between and fulcrumed on both free ends of said strips and adapted to tilt upon said strips when one strip advances ahead of the other, a circuit controller responsive to the tilting of said carrier, and a stop member for limiting the movement of one oi said strips.

14. A tire detecting device comprising two thermostatic strips having fixed ends that are movable in the same direction when said strips are subjected to elevated temperature, a tiltable carrier disposed between and fuicrumed on both free ends of said strips and adapted to tilt upon said strips when one strip advances ahead of the other, a circuit controller responsive to the tilting of said carrier, a stop member for limiting the movement of oneof said strips, and means for relatively insulating the strip of restrictive movement from the heat that influences the other strip.

15. A nre detecting device comprising two concentric parallel thermostatic spirals having free ends, a carrier disposed between and fulcrumed on both free ends, said carrier being movable with said free ends without tilting when both move together simultaneously and said carrier tilting on said ends when one of them becomes substantially displaced with respect Ato the other, and a circuit controller responsive to the tilting only of said' carrier.

1 6. A fire detecting device comprising two concentric parallel thermostatic spirals having free ends, a carrier disposed between and fulcrumed on both free ends, said carrier beingv movable with said free ends without tilting when both move together simultaneously and said carrier tilting on said ends when one of them becomes substantially displaced with respect to the other, a circuit controller responsive to'the tilting only ot said carrier, and a stop member for limiting the amount of movement of one spiral only.

17. A nre detecting device comprising two concentric parallel thermostatic spirals having free ends, a carrier disposed between and Iulcrumed on both free ends, said carrier being movable with said free ends without tilting when both move together simultaneously and said carrier tilting on said ends when one of them becomes substantially displaced with respect to the other, a circuit controller responsive to the til..ng only of said carrier, a stop member for limiting the amount of -movement of one spiral only, and means for relatively insulating that spiral from the heat which influences the other spiral.

18. A nre detecting device comprising two concentri'c parallel spaced thermostatic spirals having fixed inner ends and movable confronting outer ends, a carrier interposed between and carried by and tiltable on both free ends, and a mercury contactor tube carried by and tiltable with said carrier.

19. A ilre detecting device comprising two concentric parallel spaced thermostatic spirals hav- .ing ilxed inner ends and movable confronting outer endsz a carrier interposed between and carriedv by and tiltable on both free ends, a mercury contactor tube carried by and tiltable with said carrier, and means for limiting the movement oi the end of one spiral.

20. A iire detecting device comprising two concentric parallel spaced thermostatic spirals having xed inner ends and movable confronting outer ends, a carrier interposed between and carried by and tiltable on both free ends, a mercury contactor tube carried by and tiltable with said carrier, means for limiting the movement of the end of one spiral, and means for relatively insulating that spiral from the heat that inuences the other spiral.

21. A fire detecting device comprising a base having a recess, an insulating plate carried by said base and projecting into said recess, a pair of thermostatic spirals disposed in said recess and carried by said plate on opposite sides thereof and having free outer ends, a carrier interposed between and carried by and tiltable on both free ends, a mercury contactor tube carried by and tiltable with said carrier, stop means for limiting the free movement of the end of one of said spirals, and a heat conducting cover for 4said recess enclosing said spirals and disposed in heat conducting contact with one of said spirals.

22. A fire detecting device comprising a base having a recess, an insulating plate carried by said base and projecting into said recess, a pair of thermostatic spirals disposed in said recess and carried by said plate on opposite sides thereof and having iree outer ends, a carrier interposed between and carried by and tiltable on both free ends, a mercury contactor tube carried by and tiltable with said carrier, stop means for limiting the free movement of the end of one of said spirals, and a heat conducting cover for said recess disposed close to one only of said spirals.

23. A iire detecting device including a pair of thermo-responsive elements, a dust tight casing enclosing said elements and isolating them from the ambient atmosphere, said elements being equally freely exposed to the atmosphere within said casing, said casing having a heat-transmitting wall exposed to the atmosphere surrounding vsaid casing and adapted to become heated by exposure to the atmosphere and `to heat the interior oi' the casing, and one of said elements being disposed nearer than the other element to said wall and being in direct heat conducting contact with said wall so that it is more immediately responsive to the atmosphere externally of said casing. 4

- 24. A thermo-responsive device comprising a pair of thermo-responsive elements approximate- 1y equally exposed to receive externally applied heat and both approximately equally responsive to the same temperature, and a heated element anasmeA to the temperature of which said thermo-responsive elements are responsive, said heated element being in direct thermal contact with one thermoresponsive element to cause that thermoresponsive element to be more promptly responsive than the other to the temperature of said heated element, the contact being loose to permit movement ot the' thermo-esponsive element, and signal initiating means associated with said thermo-responsive elements.

VERNON DURBIN. 

